Fas/dpd

[chem geek]

Out of curiosity, I tried my own experiment and used 1, 2 and 4 scoops and got within 1 drop of the same results with quick consecutive tests, but I did notice variation with time -- mostly declining. There were several factors that could have contributed to the decline with time (mostly that I used a single container that was filled with pool water which was probably warming up and outgassing and getting partly contaminated even though I rinsed the measuring test tube with tap water between tests -- and yes, tap water has chloramine so that didn't help), but I kept going back to different scoop amounts and saw no variation consistent with the number of scoops. If I swirled longer I usually got a slightly lower result than expected (accounting for the slower decline with time). However, I did not try less than one scoop for a "barely visible" test.

I think the main reason that the 50 mg is recommended is that the DPD test is colorimetric in that the intensity of the pink is related to the chlorine concentration AND the amount of DPD powder, so even in the FAS-DPD test the "endpoint" of light pink to clear will be a little dependent on this absolute intensity. Taylor probably assumes a certain typical chlorine level for this test. If you only used one scoop and had very low chlorine levels, you probably wouldn't see much pink to start with so the transition to clear would be hard to catch. Also, higher chlorine levels will bleach out the DPD powder and that could be as low as 12 ppm if only one scoop were used. Two scoops is probably the minimum that works well for this test, but more scoops would certainly not be a problem as even the Taylor instructions (see this link) say to add more scoops if the pink goes away right away (which happens with > 25 ppm FC if you start with two scoops).

Anyway, it's probably best to stick with the Taylor recommendation and at least now we know that a compacted "flat" scoop fill is the accurate amount, but I'm with Carl that it's not critical to get the scoop measurement exactly right. I am not disputing your results, but it seems that there are lots of factors for variation that outweigh at least 1,2,4 scoops. Maybe more (or less) scoops than that starts to give a problem, but I didn't want to use up all my chemicals!

Richard